1. Field of the Invention
This invention relates to a direct current dynamo electric machine in which intense magnetic field poles formed by permanent magnets are arranged externally of a non-magnetic cylindrical rotor having most of its volume occupied by the winding.
2. Description of the Prior Art
In order to enhance responsiveness of a direct current dynamo electric machine, there is heretofore proposed rotors such as a smooth rotor with a winding disposed in the outer periphery of a smooth core, a print wired or printed circuit rotor having no rotor core, or a cup-shaped rotor with a winding formed into a cup-shape. The abovementioned smooth rotor is designed so that a winding is formed in the outer periphery of a core of the smooth rotor by the use of a print wiring technique or a wound winding is bonded thereto by means of an adhesive. This smooth rotor poses a drawback that in the case where the gap magnetic flux density is great, the rotor core has increased iron losses, as a consequence of which efficiency is low, and in addition, the smooth rotor suffers from a problem in terms of manufacturing technique in that securing a winding onto the core of the smooth rotor is difficult. There is another drawback in that when the winding is formed by the print wiring technique, the number of turns of winding is limited. Moreover, a high input power must be applied in order to obtain high speed responsiveness, and when the winding is thinly disposed on the core of the smooth rotor, despite the presence of such a high input power, it is not easy to fix the rotor core and the winding so as to sufficiently maintain a fixed relationship therebetween.
In the print wired rotor, a print wiring technique is applied to one side or both sides of an insulating disc to form a winding. This rotor poses a drawback that the number of turns of the winding is limited so that the efficiency is low when the rotational speed is low. Further, while the axial dimension can be reduced, the radial dimension increases in order to obtain the desired torque.
The cup-shaped rotor may be of two types, one of which is arranged such that a winding is very thinly formed into a cup-shape, whereas the other is that a winding is relatively thickly formed into a cup-shape. However, in the former type, there is a limitation in the number of turns such that it fails to enhance the efficiency of the motor when the rotational speed is low. In addition, the mechanical strength of such a rotor is small, and as a consequence, the rotor as described cannot be applied to a small output DC dynamo electric machine. In the latter type of rotor, on the other hand, the number of turns may be increased to render a relatively high efficiency at a low rotational speed, but in this case, the length of gap must be increased so that the gap magnetic flux density cannot be increased. Moreover, the cup-shaped rotor poses a drawback in that since it is supported on the shaft in a cantilever fashion, the mechanical strength is decreased.
What is therefore proposed is a cylindrical coreless rotor designed so that a rotor winding is disposed on a shaft of the rotor through a sleeve-like insulator. In this rotor, most of its volume is occupied by a winding, resulting in a large capacity of winding, thereby providing advantages such as extremely low inductance, mechanical sturdiness and low inertia. An intense magnetic field may be exerted on such a cylindrical coreless rotor to thereby obtain high efficiency, a small mechanical and electrical time constant, high responsiveness and shock-resisting input. However, since this cylindrical coreless rotor is non-magnetic, the space between magnetic field poles can be regarded as an equivalent working air gap. Thus, in order to exert the intense magnetic field on the lengthy gap as described, permanent magnets of high coercive force made of anisotropic strontium ferrite, barium ferrite or other material are well-suited. However, in the case where such permanent magnets are used to form a magnetic path having a long length of gap while being subjected to restriction in terms of form, what is called the ratio of form output, it is difficult to converge the magnetic flux of the magnetic poles in view of the ratio between the length of gap and the length of the magnetic shunt leakage in the main magnetic path. That is, most of magnetic flux are consumed as the leakage magnetic flux while the effective magnetic flux are decreased, thus producing various difficult problems in forming the actual magnetic paths.
Alternatively, in Japanese Patent Laid-open No. 50-95705 filed in Japan by The Singer Company, U.S.A., and corresponding to U.S. Pat. No. 3,891,876, there is disclosed a permanent magnet motor, in which no measure has been taken with respect to the large leakage magnetic flux inevitably produced in a magnetic field path having a long length of gap. Hence, even if permanent magnets made of rare earth alloy were to be used in this motor, the magnetic characteristic thereof cannot effectively be utilized. Further, in dynamo electric machines of relatively high output, samarium cobalt magnets are costly and permanent magnets cannot effectively be utilized. In the light of these points, the device disclosed in the abovementioned patent possesses a fatal disadvantage in terms of practical industrial use.
Also, in Japanese Patent Laid-open No. 50-125203 filed in Japan by the same applicant as that of the former (U.S. patent application bearing Ser. No. 453,738, filed Mar. 22, 1974), there is disclosed a motor in which the rotor core used in a DC dynamo electric machine is generally non-magnetized. Hence, this rotor is in the form of a coreless rotor, but most of the volume of the rotor is not occupied by a winding. The cylindrical coreless rotor imparts a gap length corresponding to the outside diameter thereof and equal to the space between the magnetic field poles, and as a consequence, the stacked density of the winding of the rotor will influence the performance.